The invention relates to a method for the manufacture of glass bodies in which a porous green body is formed from the starting material for the glass in the form of a thixotropic suspension, is dried, purified and then sintered.
The invention also relates to equipment for carrying out such a method and to the application of the glass bodies produced in accordance with the invention method.
The method referred to in the preamble is particularly suitable for the production of preforms for optical waveguides of fused silica (quartz glass).
Optical waveguides are used in a large number of applications, both for short-distance and long-distance light-transmission systems, such as in optical communication systems, and consist mainly of a glass with a high silicon dioxide content (which, where required, contains a doping agent for adjusting the refractive index of the glass).
Glasses which are suitable for optical waveguides can also be used advantageously for the manufacture of lamp envelopes for halogen and gas discharge lamps because these glasses, like the glasses for optical waveguides, have to be almost water-free and have a high silicon dioxide content.
A method is known for the manufacture of preforms for optical waveguides which starts with highly disperse colloidal SiO.sub.2 suspensions which are formed into a green body.
DE No. 30 01 792 describes a method whereby by means of a sol-gel transformation a silicon alcoholate (or silicon alkoxide)/water/alcohol solution is hydrolyzed to a gel in a container of defined shape, the gel is slowly dried and the dry gel is sintered at a temperature below its melting point. With such a method, shaping of the green body is in fact relatively simple, but drying of the gel into a monolithic green body, which has to be performed in such a way that no cracks occur in the green body, presents substantial difficulties. The drying has to be carried out either extraordinarily slowly and is therefore very time consuming or it requires considerable expenditure on equipment (e.g. drying under supercritical conditions in an autoclave). Another difficulty is that in the purification of the dry green body in a heated gas atmosphere, the gas reacts with contaminants present, on account of the very large surfaces (typical size 1000 m.sup.2 /g) and the associated very small pores between the extremely fine particles. Moreover, when green bodies produced in this way are sintered, pinhole formation is frequently observed as a result of gas molecules adsorbed at the surface and included during the sintering. Another disadvantage is the fact that it is only possible to produce green bodies with low relative green body densities of 10% of the density of compact fused silica. Such green bodies are very sensible to mechanical stresses, are therefore difficult to handle in a manufacturing process and, moreover, they undergo relatively large shrinkage during sintering.